Ultrasound, among its many applications, is used for nondestructive testing and nondestructive characterization of components and materials. It can be used for the detection of defects in components, the determination of properties of materials, the detection of thickness and proximity sensing to mention a few uses.
Many industrial manufacturing processes involve the use of high temperatures and pressures to facilitate chemical and physical reactions in the formation of materials, components and structures. Some processes involve high temperatures and corrosive environments. Some even involve thermal cycling. These conditions are often encountered in the manufacture of metals, ceramics and plastics. They are also encountered in the processing of petroleum and in the generation of energy in nuclear, fossil fuel and hydroelectric power plants. It is highly desirable to be able to monitor such processes and the structures used in the practice of such processes with the use of ultrasound. To do so, it is necessary to have ultrasound transducers that will function in these difficult environments.
Applicant's invention specifically relates to ultrasound transducers for these and other uses at high temperatures.
High temperature resistant ultrasound transducer devices are known in the art. An example is the applicant's U.S. Pat. No. 4,703,656 entitled "Temperature Independent Ultrasound Transducer Device". Other patents in the pertinent art comprise Runde et al. U.S. Pat. No. 3,781,576 entitled "High Temperature Ultrasonic Transducer"; Zacharias U.S. Pat. No. 4,505,160 entitled "High-Temperature Transducer"; Lynnworth U.S. Pat. No. 4,783,997 entitled "Ultrasonic Transducer for High Temperature Applications" and Light et al. U.S. Pat. No. 5,195,373 entitled "Ultrasonic Transducer for Extreme Temperature Environments".
A persistent problem with certain of the high temperature ultrasound transducer devices is maintaining intimate contact between the piezoelectric element and the protecting or delay block to which it is secured. The adhesives available for making the contact deteriorate at high temperatures and under ultrasound induced conditions. Some commercially available ultrasound transducer devices use organic epoxies for bonding the piezoelectric element to a protecting or delay block comprised of a high temperature resistant polyamide plastic. Even at a temperature of about 200.degree. C., bonds between the piezoelectric elements and the plastic protecting or delay blocks separate. Moreover, the plastic delay blocks themselves deform when subjected to a temperature of about 500.degree. C. It should be understood that while the transducer devices are placed into contact with very high temperatures, the temperature of the piezoelectric elements themselves must not exceed the Curie point (temperature) of the elements at which temperatures the piezoelectric properties are lost. This is achieved by maintaining a temperature gradient between the component or process to which ultrasound pulses are being applied and the piezoelectric element.
Mechanical clamping has been suggested to secure the piezoelectric element to the protection or delay block. However, mechanical clamping itself has certain drawbacks relating to the ability of the transducer to produce pulses useful in testing applications. It is necessary that the ultrasound pulses of selected frequency distribution and pulse width be transmitted without undesirable echoes and/or attenuations resulting from the transducer structure itself.
It is an object of this invention to provide a high temperature resistant ultrasound transducer device that can be configured to provide a narrow ultrasound pulse having a frequency between less than 0.25 to greater than 10 megahertz at contact face temperatures up to about 1500.degree. C. for short times and at lesser temperatures for longer times.
It is a further object of this invention that the premature failure of the bond between the piezoelectric element and the delay block is eliminated by a mechanical structure that holds all components in place while permitting the piezoelectric transducer to generate pulses of desired frequency, frequency distribution and pulse width without undesired echoes and/or attenuations.
It is a still further object of this invention that the pulse width and attenuation characteristics of the transducer devices are not unacceptably reduced at elevated temperatures and delay times remain stable over long periods of time.